1. Field of the Invention
The present invention relates to sensitizers for heat sensitive leuco dye-based recording papers.
2. Description of Related Art
Direct thermal printing of documents offers numerous advantages over other printing methods due to its simplicity and quietness and has been widely used for chart recorders and telecopy machines. As with any hardcopy generation method, it is desirable that thermal printing be as fast as possible. To increase the speed of thermal printing, shorter dwell times between the thermal printing head and the heat sensitive paper must be achieved. To reduce print head dwell time, either more energy must be delivered to the paper, and thus to the print head, in a given time, or the sensitivity of the paper must be increased. Input energy to the thermal head is limited since the energy shock caused by a short heat/cool cycle deteriorates the life of a thermal head very quickly. Therefore the sensitivity of the heat sensitive recording paper must be increased.
Various methods have been attempted to increase the heat sensitivity of the paper. Calendering and precoating the paper before application of heat sensitive compounds are two techniques used for this purpose. The calendering smooths the surface of the heat sensitive paper for better contact with the thermal printing head, and the precoating inhibits the heat sensitive compounds from soaking into the paper. Although these methods have increased the sensitivity of the paper, even higher sensitivity is desired.
Recent efforts have focused on increasing the sensitivity of the heat sensitive paper by increasing the sensitivity of the dye precursor and developer combinations used in the heat sensitive compounds on the paper. Dye sensitization has been attempted previously, especially in the field of photosensitive diazo dyes. Diazo dyes are based on the diazo group (--N.dbd.N--). Photosensitizers for diazo dyes work by forming a charge transfer complex with the diazo dye precursor.
The dyes employed in this invention are not diazo dyes. Specifically, the dyes of interest here are leuco dyes having a five-membered spirol lactone ring at one end of the molecule, and a tertiary amino group at another end of the molecule to facilitate a lactone ring opening. Leuco dye precursors are colorless or light colored. In such a system, the heat sensitive coating comprises a dye precursor and a developer. The image is formed by the chemical transformation of the dye precursor into a dye by reaction with the developer. This transformation results in a visible product. The dye itself produces the visible image. Heating the coating allows the developer and dye precursor to react and form color.
Low melting waxes and fatty amides have been used to increase the sensitivity of thermal paper by lowering the melting point of the heat sensitive coating. Such waxes and amides can cause background instability and residue build-up on the thermal head and are therefore generally not preferred.
A number of sensitizers have been found to improve heat sensitivity of the paper without the drawbacks of the waxes and fatty amides. Some of those compounds are aromatic ethers, aromatic esters, or biphenyl derivatives, including 2-benzyloxynapthalene, 1,4-diphenoxybutanes, 4-benzylbiphenyl, and o-acetoacetotoluidide. Sensitizers facilitate the dye forming process. The thermal sensitizers are believed to function by lowering the eutectic melting point of the dye precursor/developer system, or by acting as a solvent in which a dye precursor and developer dissolve below their melting point.